It is known to use electromagnetic relays for low-power switching of electric loads. A switched electric load may e.g. be an inductive or a capacitive load, for example in an industrial application or in a motor vehicle. However, electromagnetic relays have the disadvantage that they require a lot of space and switch relatively slowly. Additionally, electromagnetic relays consumer relatively large amounts of electrical power and are prone to failure.
Attempts have been made to avoid these disadvantages by using electronic switches, in particular transistors, instead of electromagnetic relays to shorten switching time and to reduce power consumption. These are the so-called solid state relays. However, known solid state relays have the disadvantage over electromechanical relays that they can switch current in the output circuit only in one direction. Also, input and output circuits are vulnerable to overload and interferences and have higher voltage drops in the output circuit than traditional relay.
Due to the fact that known solid state relay can only switch unidirectionally, it is not possible to use them without paying close attention to the polarity of the circuit in which they are used. Rather, there is a risk that the transistor is destroyed. Also, an improper polarity reversal can lead to damage or destruction of the transistor, especially at relatively high currents.
There is therefore a vital interest in developing solid-state relays which have the same beneficial characteristics as electromechanical relays. So far, this has not been possible. Therefore, electromechanical relays cannot be easily replaced in a variety of applications.
Classic high-power switches such as thyristors or TRIACs are known in the art which are capable of bidirectional switching. However, they have the disadvantage that they cannot be turned off via the gate. Only after a polarity reversal of the switched voltage do they return to a high-impedance state and can be re-triggered. Newer technologies such as GTOs or IGCTs can be turned off through the control gate, but due to their package and relatively high cost they are not practical for a variety of applications.
It is therefore an object of the disclosure to provide an electronic switching circuit, in particular a solid state relay, which can switch comparatively high currents bidirectionally.